How to add Wear & Tear effect in a Texture
Wear and tear detail on the surfaces of the models can be painted in using bump maps that are either imported or created using Blender's Texture Paint tools. Procedural textures also work well for this.

Start with a grayscale image(set to non-color) and connect this to Displacement socket of the Cycles Material output node.



The bump map node does not currently work for baking normal maps in Cycles.  Only use the displacement socket for now when baking.



Next, bake the texture so it can be used in the game engine this is supposed to be for(I'm making an assumption that this is what you are doing ;) Be sure to select the texture node that is being baked to so that the baker tool accesses the proper texture.  It should be active in the editor.



Now test the results by plugging the normal map into the shaders' normal socket.





You can bake normal maps using either Blender Render(Internal) or Cycles.  For many situations the results should be identical.  

With Cycles, there is the additional option of using a custom defined cage when working with difficult geometry.